Field of the Invention
This invention relates to a preburner furnace and more particularly, to a preburner furnace used to fire a boiler, and to a combination of a preburner furnace and a boiler.
A boiler which is designed to be fired by oil and/or gas commonly comprises a water cooled combustion tube, closed at one end and located within a water space of the boiler, an oil and/or gas burner being mounted in a panel such as a door at one end of the combustion tube, the flame being projected into the combustion tube. Upon reaching the closed end, the flame reverses direction and passes from the combustion tube into a smoke box at the end of the combustion tube adjacent the door, from where the flame, hot gases and other combustion products (herein referred to as hot gases) pass into water cooled smoke tubes, which extend through the water space of the boiler, and are eventually directed to a flue.
Boilers or furnaces which are fired by oil and/or gas have a positive flame characteristics in that they do not need a draught to induce the hot gases into the smoke tubes because the oil and/or gas is injected or introduced respectively, into the boiler or furnace continuously under pressure. However, boilers or furnaces which are fired by solid fuel normally have a negative flame characteristics in that it is necessary to induce a draught through the burning solid fuel.
Previously, converting a boiler which is specifically designed for oil and/or gas firing to solid fuel firing and achieving the rated output, has proved difficult for the following reasons.
First, a combustion tube of an oil and/or gas fired boiler is generally physically smaller than the equivalent solid fuel fired boiler for a given output as the calorific value of oil or gas is greater than solid fuel, and thus it is not necessary to burn such a great quantity of oil or gas at once.
Secondly, there is not a sufficient space to provide a fire bed in the combustion tube of an oil and/or gas fired boiler. If solid fuel were burnt in the smaller oil and/or gas fired boiler combustion tube, at least some of the combustion tube wall surface will be covered by the fire bed thereby reducing the effective wall area of the combustion tube which the hot gases can heat. Thus the efficiency of the boiler will be reduced and the boiler will not be able to achieve its rated output i.e. the boiler will be de-rated.
Thirdly, in solid fuel fired boilers it is normally necessary for solid fuel to be burnt along substantially the entire length of the combustion tube because, if solid fuel is only burnt at one end e.g. adjacent the door, the flame and hot gases produced would pass directly into the smoke box at that end and from there into the smoke tubes without satisfactorily heating the entire length of the combustion tube or reversing direction at the closed end thereof. Thus it is difficult to direct solid fuel to the closed end of the combustion tube of a boiler designed for oil and/or gas firing, and thus to ensure that the solid fuel is burnt along substantially the entire length thereof.